The present invention relates to a fuel inerting system, and in particular to a system that combines both fuel inerting and fuel scrubbing techniques.
It is vital to minimise the risk of fire in the fuel tanks of aeroplanes. This is particularly important in military aircraft where there is a fire risk during battle. The risk of a fire is substantially reduced by providing a flow of inert gas, usually Nitrogen Enriched Air (NEA), into the ullage space above the fuel in each fuel tank. Ullage, for the purposes of this specification, refers to the part of a fuel tank that is not filled with fuel. It is also important to carry out a process known as scrubbing in which dissolved oxygen in the fuel is displaced by mixing NEA with the oxygenated fuel.
One method of scrubbing fuel is to use the motive force of the fuel entering a tank during refuelling to power a centrifugal separator, or another type of mixer, using jet-pump action. This draws the NEA from the tank ullage in order to displace oxygen from the fuel. An alternative method of scrubbing is to introduce pressurised NEA directly into the tank.
The present invention provides a fuel inertion system that combines the two aforementioned methods, thereby providing a single inertion and scrubbing system which can be employed in either condition.
Accordingly, there is provided, a fuel inertion and scrubbing system, comprising a first supply means to supply an inert gas to a fuel tank to scrub the fuel therein when the ullage of the fuel tank is inert, and a second supply means to supply the inert gas to scrub the fuel therein when the ullage of the fuel tank is not inert, the first and second supply means being individually operable by means of a valve arrangement, in such a manner that only one of the supply means is operable at any given time.
In such a combined system, when the ullage is inert, for example during normal air-to-air refuelling, scrubbing of the fuel as it enters the aircraft is carried out using a mixer, for example a jet-pump mixer, drawing NEA from the ullage. However, when the ullage is not inert, for example during initial fuelling for a tactical military mission, scrubbing of the fuel is carried out using a pressurised NEA supply. The operation of the two systems is selected by a valve arrangement.
Current systems, like that used, for example, on a Boeing C-17 aircraft, incorporate a nitrogen separator and a storage system for the inert gas, both of which are fixed within the aircraft structure. Pressurised air from the aircraft engines is then used as a supply to the nitrogen separator.
The present invention also provides a system that incorporates a separator and an inert gas storage system which is carried in the aircraft cargo bay or other pressurised area of the aircraft, and which can be removed when the system is no longer required. The present system takes air from the aircraft pressurised cabin via the cargo bay, or other pressurised area, and compresses it using a separate compressor before supplying the air to the separator. As a result of this portable system, it is possible to operate a separator that is normally ground-based in flight at altitude.
Preferably, the first supply means consists of a jet-pump mixer drawing the inert gas from the ullage, and the second supply means consists of a supply of pressurised inert gas.
Preferably, the inert gas is Nitrogen Enriched Air.
The valve arrangement preferably consists of a pressure-actuated priority valve operable so as to shut off flow of the inert gas from the ullage to the jet pump mixer.
An oxygen sensor is preferably provided within the ullage of the fuel tank. The oxygen sensor measures the oxygen content of the gas in the ullage and from this, the inertness of the ullage area can be found.
The pressurised inert gas supply can be switched on and off either manually or automatically.
It is favourable for the system to further comprise a nitrogen separator and an inert gas store which take their air supply in flight from the aircraft pressurised cabin, and are portable and can be removed from the aircraft when no longer required.